The present invention relates specifically to high current semiconductor devices and more particularly to a power lateral PNP device using a buried power buss.
Lateral PNP transistors are utilized extensively in high power applications. They are typically deeply diffused devices that carry very high current (1-5 amps or higher). FIG. 1 shows a cross-section of a standard deep diffused PNP device 10. The emitter 14 is the inside electrode. As shown, this can represent a bulls-eye pattern with the emitter as the bulls-eye and the collector surrounding it, or double collector lines adjacent to each side of an emitter. The device 10 includes two P+ collectors 12a and 12b and P+ emitter 14 between. The two P+ collectors 12a, 12b and the P+ emitter 14 are diffused in an N epitaxial layer 15. An N+ layer 18 (the buried layer) is deposited in a Pxe2x88x92 substrate 20 which is coupled to the N epitaxial layer 15. An N+ base contact 16 is coupled to the surface of the N epitaxial layer 15 and coupled electrically via the N buried layer to apply a voltage between the N+ base and the P+ emitter 14. The collectors 12a and 12b include a metal layer 17a and 17b respectively on the surface thereof. The collectors TCF 12a and 12b are outside circular electrodes when a bulls-eye pattern is used and are parallel separate structures when parallel inline design is used. The base 16 is contacted via a metalized N diffusion that is placed in the N epitaxial layer 15. For some applications it is tied to the N+ buried layer.
Injection from the emitter 14 is from the total outside periphery of the emitter for the total depth of the emitter. This results in a tremendous difference in the base widths since the surface portion of the emitter 14 is closest to the edge of the surface of the collectors and therefore has the shortest base width. Moving down the periphery of the emitter 12a and 12b, the base width becomes longer and longer and reaches its maximum base width at the deepest point 23.
Referring back to FIG. 1, it is obvious that most of the injection and collection could be considered coming from two transistors in parallel. Transistor XR being on the right half of the emitter and its injection being collected by transistor XR While at the same time transistor YL on the left half of the emitter has its injection being collected by YL, the collector on the left side. To give some general quantitative idea of the basewidths, assume that the distance from the emitter to collector is 8 xcexcm on the mask. If, for example, the P diffusions are 2.5 xcexcm deep and the side diffusion is 2.0 xcexcm around the total periphery of the emitter and collector, then this leaves the basewidth approximately 4 xcexcm long at the surface and approximately 8 xcexcm long from point 23. In fact, these effective basewidths are much less due to the depletion region extending from the collectors into the base region 16 (N epitaxial layer) and the depletion region of the emitters extending into the base region 16 (N epitaxial layer). This particular example may not be able to work at very high voltages due to punch-through. It is very easy to have depletion widths of a micron or more. This leaves the surface with a basewidth of approximately 2 xcexcm and the basewidth of the bottom of approximately 6 xcexcm.
Without surface effects, this means the surface portion of these two transistors in parallel have the highest beta and the best frequency response, while the deep points have the lowest beta and the worst frequency response.
In general, the beta coming from the bottom point 23 of these transistors can be ignored. Beta is much lower than is achieved at the surface mainly limited by recombination in the bulk as well as the fact that the base emitter voltage (Vbe) is somewhat less at the bottom due to some additional drop from the base contact to the actual base. Likewise, it can be assumed that some of the surface beta is lost due to surface recombination velocity. It can then be assumed that the beta is the average beta with a base width of approximately 3 xcexcm. However, beta is a function of the amount of current collected versus the amount of current emitted. The current being emitted is along the total periphery of the two transistors in parallel as determined by the base emitter voltage (Vbe) and the resultant low base current. The current being collected as a result of this emission is much less due to the issues just discussed, therefore resulting in a beta that is much lower.
The frequency response of the standard lateral PNP is determined by the worse response of the structure. This means the bottom of the radial structure is determining the frequency response of the device due to its long basewidth Frequency response is determined by where the output is down to 0.707 of the low frequency output. At low frequency the current, and therefore the beta is made up of all the varying basewidths from top to bottom of the structure.
As the frequency increases the bottom of the structure with the long basewidth has recombination occurring on the long basewidths and the output current for a given input current goes down. This shows the total structure as having a lower output current as the frequency increases. The long basewidth device is therefore determining when the overall output is down to 0.707 of the low frequency output For a power lateral PNP device, where frequency response may not be an issue this is of secondary concern; gain is the primary concern.
Another issue with the standard approach relates to debiasing of the emitter when carrying high current. This occurs because metal is on top of the emitter and therefore the maximum voltage is applied on the surface and the voltage drops to lower values as one goes along the depth of the emitter due to drops in the resistance of the emitter. This drop can be very high since the current may run lamp to greater than 5 amps and any amount of resistance will result in significant debiasing.
An ideal lateral PNP would have a profile as shown in FIG. 2, where the emitter and collectors are vertical spikes that have the same basewidth from top to bottom. It would also have metal the full depth to reduce debiasing.
Accordingly, what is needed is a system and method for providing a power lateral PNP that approaches this ideal structure. This lateral PNP would have an improved beta and frequency response. The present invention addresses such a need with two approaches.
A power lateral PNP device is disclosed which includes an epitaxial layer; a first and second collector region embedded in the epitaxial layer, an emitter region between the first and second collector regions. Therefore slots are placed in each of the regions. Accordingly, in a first approach the standard process flow will be followed until reaching the point where contact openings and metal are to be processed. In this approach slots are etched that are preferably 5 xcexcm to 6 xcexcm deep and 5 to 6 xcexcm wide. These depths are examples. They can be changed for different thicknesses of epitaxial material or for different junction depths in the given technology. These slots are then oxidized either thermally or by deposition of an oxide or dielectric layer and will be subsequently metalized. When used for making metal contacts to the buried layer or for ground the oxide is removed from the bottom of these particular slots by an anisotropic etch. Subsequently when these slots receive metal they will provide contacts to the buried layer where this is desired and to the substrate when a ground is desired.
In a second approach the above-identified process is completed up through the slot process without processing the lateral PNPs. With a separate masking and etching, the oxide is removed from the PNP slots and boron is deposited in a diffusion furnace and driven in a non-oxidizing atmosphere.